 Welcome to this lecture number 7 on continuation of this aquifer classification. So, we had in the previous lecture we are discussing about the aquifer characteristics and classification and specifically we discussed about say two types of aquifer. The first one is the unconfined or table aquifer and we also discussed about confined aquifer which is also known as artesian aquifer or pressure aquifer. This unconfined aquifer is having an impervious stratum only at the bottom and it has a variable top, it does not have any confining layer and water table which is variable depending upon various factors. So, that forms the upper flexible boundary of this unconfined aquifer. On the other hand, so this confined aquifer, it is known as artesian aquifer or pressure aquifer. It has two confining layers one at the top as well as the one at the bottom as shown here and so here this water the ground water will be under pressure much above the atmospheric pressure that is why it is also known as pressure aquifer or artesian aquifer. Now, let us also learn about say two more aquifers. So, the first one is the perched aquifer. So, this perched means a raised aquifer. Essentially, it is a locally raised aquifer. Suppose in a particular area below the ground, there is a small saucer like impervious layer for a limited extent and over this impervious layer some amount of ground water gets stored. So, this represents the water table and so here this represents a zone of aeration and below that so this represents a zone of saturation. So, this one represents the perched aquifer. So, the perched actually so I would like to say this is the perched water table and this one is the perched aquifer. So, here we should bear in mind that the impervious layer let me write it as a localized impervious layer has a very limited aerial extent and therefore, the amount of ground water stored in a perched aquifer is also limited and therefore, it is available only for a small period of time. And of course, because so above this perched water table, there is zone of aeration and below the perched water table, there is zone of saturation. So, it also is included as one of the maybe here we can call it a localized aquifer localized unconventional aquifer. So, that is what a perched aquifer represents. So, this forms the third type of aquifer and the last the fourth and the last type of aquifer that we are going to discuss here is the leaky aquifer can also be called semi confined aquifer. In this case it has it is a confined aquifer the only difference is one of the layers either the bottom layer and in this case the top layer. So, this is a impermeable top layer. So, this is a semi permeable bottom layer or it can be the other way also and of course, so this is the other name for the semi permeable bottom layer is aquitard. This is the leaky aquifer it can also be the other way also that is the top layer may be semi confined semi permeable or aquitard and the bottom layer. So, in this case, so this is the top aquitard that is semi permeable layer and this is the bottom semi permeable or semi pervious impervious layer and here this is the leaky aquifer also known as semi confined aquifer. So, in this case since the top layer is semi pervious permeable or say pervious. So, from the top layer there will be a slow contribution of ground water into this leaky aquifer slow ground water contribution from top. Likewise as mentioning here the semi permeable or say semi pervious bottom layer in this case there will be, so this represents slow ground water leakage at the bottom. So, it is for this reason, so these are known as semi confined aquifers. So, basically here you can say it is on one end either at the top or at the bottom there is a confine there is a confining layer which is fully impervious and the other end the either in this case here it is at the bottom and in this case here it is at the top. So, the layer is an aquitard which is semi pervious. So, these the unconfined aquifer or the water table aquifer the confined aquifer or the artesian or pressure aquifer and thirdly the perched aquifer or raised aquifer we may also refer to as a localized raised unconfined aquifer and fourthly it is leaky aquifer or say semi confined aquifer. So, these are the four types of aquifers each one of them will be a will yield ground water depending upon the various factors and among this one of the important factors which determines the this the ground water yield from an aquifer is known as the storage coefficient also known as storativity. And this storativity is the volume of water you can say volume of ground water that an aquifer releases or absorbs per unit surface area unit change in head. Obviously, head is measured perpendicular to the surface area. So, this is the known as a storage coefficient of storativity and generally. So, the for confined aquifer and this is denoted by the letter S generally for confined aquifers it is a unit less basically because it represents ratio of two volumes for confined aquifers. So, this is the storativity will be in the range of 5 into 10 to the power minus 5 to 5 into 10 to the power minus 3. So, this is the storage coefficient and it the specific yield or the total ground water yield from an aquifer very much depends upon this parameter. And so, this storage coefficient is considered as one of the three important formation constants of an aquifer the other two being the hydraulic conductivity or permeability and transmissibility or transmissibility which of course, we will discuss sometime later in this lecture. Now, we will discuss little bit about the ground water basins and springs. So, just like the water basin which is essentially represents a particular area on the surface of earth which holds water and there will be specific drainage pattern. So, in this case a ground water basin is also some kind of physical entity which has a certain aerial extent and this ground water basin consists of one large aquifer as well as the number of small aquifers. Just like in the surface water basin there will be a main course main stream channel or main course and it is there will be a number of other courses. So, in this case also in case of ground water. So, there will be a main aquifer and a number of interrelated aquifers. So, here we can this ground water basin. So, we may define it as a hydrologic or hydrogeologic unit containing a large aquifer and a number of other interconnected aquifers interconnected or say connected aquifers and like the surface water basin or catchment which is also referred to as water shed. So, this ground water basin also has storage and this transport ground water transport both are involved and many times. So, this ground water basins consist of a large area, aerial extent as well as depth which can yield a significant amount of ground water. Now, let us come to the springs. So, the spring is it is basically a water gushing out from the ground surface is generally known as a spring. So, we can write this as a concentrated discharge of ground water appearing at ground surface as a current of flowing water. So, here you can say spring it is basically at the ground surface. It represents an interface and upstream of the spring the water is in ground water form and downstream of the spring the water is in water comes on earth and so it is a surface water form. And these springs may be of different types such as here you can say this is types of springs and of course, so this springs in this the water may come out as a current either in the normal temperature or it may come out as a current of a hot or higher temperature. So, like this, so the spring can be either a normal water spring or a hot water spring. So, in this in the normal water spring the temperature is the normal temperature and which are generally referred to as simply springs. So, these are let me write here. So, this is the normal temperature springs. So, the various types let us discuss few of them among the normal temperature springs is the depression springs followed by contact springs followed by rotation springs or say fracture rotation springs then there is a tubular spring. So, these are the 4 important types of springs. Now, let us discuss briefly about each one of them. So, here let us consider say suppose this is a ground water this is a subsurface layer and in this say let us say, so this is the ground level and then this is the water table. So, here at this point the water table meets the ground level and so therefore, here this we have what is known as a depression spring through which the water gushes out from and suppose there is some as normal even if we create a very small depression. So, the water gushes out through there. So, this is the first type of spring next we have what is known as the contact spring and in this contact spring suppose we have a ground water amount and width. So, this is an impervious layer and here it is overlaid by a pervious layer and in this. So, this is a and this represents the water table and this water table is in contact with the ground profile at these 2 points where we have we will get a contact spring. So, this is again a. So, essentially here what happens is, so the water slowly will gets released from this contact spring and then it flows along the impervious layer of this soil or rock mound and so eventually, so it may join a stream or it may join a lake or any other surface storage area like that. So, essentially this contact spring is found and of course, in this case unlike the the depression spring. So, generally the water the current velocity will be slightly less. Now, the third one let us is the fracture artesian spring wherein we have suppose a pervious layer which is overlaid by an impervious layer. So, this is a impervious layer again this is also an impervious layer in between there is a pervious layer which is an aquifer. So, this is a and there is a fracture. So, here this you can say this is a fractured rock over the impervious layer which is lying above the impervious layer and through this fracture the water gushes out through what is known as fracture artesian spring. So, essentially so this is a there is a fracture in the impervious layer which is over and above the aquifer and through this fracture so the water gushes out through what is known as the fracture artesian spring and the last one. So, these are the three types of springs. The last one let us discuss is the tubular spring and in this tubular spring we have suppose this is the a fractured rock. So, these are the fractures in the rock and suppose this is the level up to which the ground water is stored and obviously the same level is maintained here also and here at this point so this is a tubular spring and here you can say so these are the fractures saturated with ground water and of course so this is the ground level. So, because here the fracture continues up to the ground and then the ground water stored in these fractures the level of ground water stored in these fractures is above this the level of tubular spring so the water gushes out as a spring. So, these are the four types of springs and now let us also discuss about the high temperature springs which are known as say thermal springs they are also known as geothermal springs. So, in this what happens is so the because of the large temperature the hot temperature through this cooling magma chamber. So, this is the heat getting released and what this does is suppose this is the so here the surface water gets released so this is the descending cool surface water and here of course have a so this descending cool surface water comes in contact with the heat released through the cooling magma and then here so this is the so this is the level of water and this is the rising hot water. So, when this descending cool water cold water cold surface water comes in contact with the heat released by the cooling magma chamber. So, its temperature gets increased and then it comes as a rising hot water. So, here so this is the hot spring or geyser and obviously so this there will be many so this cooling magma chamber is at a great depth something like say 3000 meters or so and because of that so the thermal springs and many times so they will also have say other mineral ingredients and so these cooling springs are available in are can be these hot water springs or say thermal springs or geothermal springs can be found in different parts of the world such as say New Zealand there are ample amount of sun and many times so this geothermal springs are also used in this generating electricity. So, these are some of the springs so initially we saw the four normal temperature springs or simply the springs and followed by we also discussed briefly about the geothermal or say thermal spring or hot water spring. Now, so this is so far I have discussed about the ground water storage or occurrence part of it. Now, let us discuss little bit about the Darcy's law which essentially represents the basic equation or basic law governing the ground water movement. So, here so this Darcy's law we can state this as the discharge Q is equal to so firstly it was stated as the discharge is proportional to I into A. So, this Q is the discharge so here this is the ground water discharge and this I is known as the hydraulic gradient hydraulic gradient and this A is known as the is the cross sectional area of flow of ground water flow. So, when Q is proportional to I into A the product of I and A so we can as well write this Q is equal to K I A where K is the proportionality constant which is known as permeability. It is also known as the coefficient of permeability or it is also popularly known as hydraulic conductivity. So, here we can write this down as a Q over A which is equal to K into I. So, this Q over A Q over A is also equal to the velocity and in this case it is the here we can take it as the apparent ground water seepage velocity and here the left hand side has the dimensions of velocity. So, for this equation to be dimensionally homogeneous so the dimensions of this K which is the coefficient of permeability or it is also simply known as permeability or hydraulic conductivity is equal to the dimensions of velocity divided by the dimensions of this hydraulic gradient and this hydraulic gradient is a pure number with no units. So, therefore, this hydraulic conductivity or permeability or coefficient of permeability has the units of velocity as the dimensions as well as units of velocity. So, this is Darcy's law which was stated by the French hydraulic engineer Henry Darcy in say 1856. So, here this I which is the hydraulic gradient so this is equal to minus d H by d L. So, there is a negative sign to indicate that as the travel distance the ground water travel distance L increases the head the ground water head or H decreases. So, therefore, so this I will have a negative sign. So, therefore, here we can mention we can state here. So, this is V is equal to minus K into d H by d L. So, this is which is equal to simply K into I. So, this relationship is popularly known as Darcy's law which is applicable for say ground water flow and as you can see. So, this ground water flow it is a laminar flow it is a highly laminar flow with the velocity is very small. So, therefore, so the this ground water velocity for which the Darcy's law is applicable. So, it depends upon the parameter which governs the laminar flow that is the Reynolds number here we can denote this as R e. So, this for the so the Darcy's law is valid for Reynolds number let me say perfectly valid for Reynolds number less than 1 and here we know this Reynolds number it is defined as the ratio of inertia force to viscous force and mathematically it is equal to rho V d by mu where rho is the density V is the velocity D is the characteristic dimension and mu is the viscosity or the coefficient of viscosity and. So, for Reynolds number greater than 1 and less than 10. So, the Darcy's law is more or less applicable more or less that means it is almost valid. So, only when the Reynolds number exceeds this 10 which is very rare in case of ground water flow. So, then so this the Darcy's law will not be valid and also we know that as per the Hagan Poiseuille equation for the hydraulic conductivity. So, this coefficient of permeability can be expressed by C d m square into gamma by mu. So, this is by analogy with Hagan Poiseuille flow equation for laminar flow. So, this coefficient of permeability. So, here the C is the shape factor and this d m is the mean particle size and this gamma is the specific weight of water fluid and of course, in this in case of ground water it is a water and obviously. So, this mu is the viscosity and so this is equal to rho into g where rho is the density of water. Now, so here we can conclude that so this K is equal to C d m square and this gamma which is rho into g and this g if we express it as the denominator of the denominator. So, this will be this is g and I am sorry this gamma if I express it as denominator of denominator. So, then so this is the kinematic viscosity which is denoted by the small letter nu. Therefore, the hydraulic conductivity or the coefficient of permeability or permeability is inversely proportional to the kinematic viscosity of water. So, as the kinematic viscosity changes. So, the hydraulic conductivity varies inversely. So, here this term the C d m square it is denoted by K 0 which is the which is known as the intrinsic permeability. So, obviously, so this C d m square it has nothing to do with the fluid properties, it has only to do with the properties of the flow medium which is soil or rock in this case. So, here so this intrinsic of course, so this intrinsic permeability this is also known as a specific permeability is related with the permeability by the equation K is equal to K 0 into g divided by nu. So, this and the dimensions of this intrinsic permeability has the dimensions of this dimensionless constant as well as d m square. So, it has the dimensions of say length square. So, this is the intrinsic permeability which has so this is a function of the flow medium only. And the next class we will discuss about this transmissibility or transmissivity as well as other. So, this transmissivity along with this hydraulic conductivity and storativity forms what are known as the formation constants of aquifers. So, in the next lecture we will discuss further about this hydraulic conductivity, transmissivity and its determination and other related topics. Thank you.